The present application claims the priority of Japanese Patent Application No. 2001-012600, filed in Japan on Jan. 22, 2001, the entire contents of which are hereby incorporated herein by reference.
1. Technical Field of the Invention
The invention relates to a method of cleaning a chemical vapor deposition system(referred to in the present patent specification as xe2x80x9cCVD systemxe2x80x9d).
2. Description of Related Art
It is known to use high-temperature polysilicon-type TFTs (thin-film transistors) and low-temperature polysilicon-type TFTs in methods of manufacturing liquid crystal displays.
In order to obtain high-quality oxide films in the manufacturing methods using high-temperature polysilicon-type TFTs, quartz substrates which could withstand high temperatures of 1000xc2x0 C. or more are used.
In contrast, in the manufacture of low-temperature polysilicon-type TFTs, it is necessary to carry out film deposition in a low-temperature environment (for example 450xc2x0 C. or less) because a glass substrate which is customary for TFTs is used. Methods for manufacturing liquid crystal displays using low-temperature polysilicon-type TFTs have the advantage that they do not require special substrates to be used. Such methods have been put into practice in recent years and their production volume is continuing to expand.
In the manufacture of liquid crystal displays using low-temperature polysilicon-type TFTs, plasma CVD is used when a silicon oxide film is deposited as a gate insulator film at low temperature. When silicon oxide film is deposited by plasma CVD, silane, tetraethoxysilane (TEOS) and the like are used as typical materials in gas form.
If silane, or the like, is used as the material in gas form and silicon oxide film is deposited by means of plasma, in the conventional plasma CVD system, silicon oxide film is deposited on the surface of a substrate by introducing the material in gas form and oxygen, or the like, into the space in front of said substrate, generating plasma in a gas mixture comprising the material in gas form and the oxygen or the like and exposing the substrate to said plasma.
The conventional plasma CVD systems are configured in such a way that the material in gas form is supplied directly into the plasma which is generated inside the plasma CVD system. For this reason, with the configuration of conventional plasma CVD system, there is a problem that the high-energy ions are injected from the plasma present in the space in front of the substrate onto the film-depositing face of the substrate and they damage the silicon oxide film and degrade the properties of the film. Furthermore, as the material in gas form is fed directly into the plasma, particles are produced by violent reaction between the material in gas form and the plasma, and as a result the yield is reduced.
In order to solve the above mentioned problems, an attempt to improve the CVD device of the remote plasma type is disclosed in Japan Patent Application Serial Number H11-157692.
The CVD device disclosed in the above mentioned the patent application, Serial Number H11-157692, produces active seeds (radicals) by generating plasma inside a vacuum vessel, carries out the film-deposition processing on a substrate, accommodated inside said vacuum vessel, by means of these active seeds and material in gas form.
That is to say, an electrically conductive partition plate which divides the interior of said vacuum vessel into two chambers is provided in said vacuum vessel. The interior of one of these two chambers is formed as a plasma-generating space in which high-frequency electrode are arranged, and the interior of the other chamber is formed as a film-deposition processing space in which a substrate-holding mechanism on which a substrate is mounted is arranged. A plurality of through-holes which are made to pass from the plasma-generating space to the film-deposition processing space are formed in this electrically conductive partition plate. Furthermore, this electrically conductive partition plate has an interior space which is divided off from the plasma-generating space and communicates with the film-deposition processing space via a plurality of diffusion holes. The system is configured in such a way that the material in gas form is supplied to the interior space of this electrically conductive partition plate from the outside and fed into said film-deposition processing space through said plurality of diffusion holes. The active seeds which are generated in said plasma-generating space are fed into the film-deposition processing space through the plurality of through-holes formed in said electrically conductive partition plate and film processing is performed on said substrate in film-deposition processing space.
In said CVD system disclosed in Patent Application Serial Number H11-57692, the plurality of through-holes which are made to pass from said plasma-generating space and are provided in said electrically conductive partition plate to said film-deposition processing space are formed to satisfy the condition uL/D greater than 1 when the gas flow velocity inside said through-holes is u, the effective length of the through-holes is L and the coefficient of mutual gas diffusion is D.
As the plasma-generating space and film-deposition processing space are separated by means of the electrically conductive partition plate in said CVD system proposed in Patent Application Serial Number H11-157692, the device is configured in such a way that the processing surface of the substrate which is arranged in the film-deposition processing space is not exposed to the plasma. In addition, a plurality of through-holes which are made to pass from the plasma-generating space to film-deposition processing space are formed in the electrically conductive partition plate. However, because these through-holes are formed so as to satisfy the above_mentioned condition, the material in gas form which is fed into the film-deposition processing space is prevented from diffusing back into the plasma-generating space.
It is to be noted that in Patent Application Serial Number H11-157692, a CVD system is proposed which is formed in such a way that said plurality of diffusion holes also fulfill the above mentioned condition placed on the through-holes, in order to prevent the active species fed into the film-deposition processing space from diffusing back into the interior space of the partition plate.
In fact, Patent Application Serial Number H11-157692 discloses a CVD system in which plasma is generated between the high-frequency electrode and the lower face part of the upper part of the vacuum vessel and in the space which is bounded by the high-frequency electrode and the partition wall comprised of vacuum vessel which makes up the CVD system and the electrically conductive partition plate, both of which are at ground potential. Further more, the variation of the above mentioned CVD system is disclosed in which, the high frequency electrodes are installed in upper positions in the plasma-generating space and plasma electrical discharge is produced between the high-frequency electrode and the electrically conductive partition plate.
Generally, there are problems common to CVD systems that when films continue to be deposited, they are also deposited on the substrate-supporting elements and the interior wall of the film-depositing chamber and the like. When they drop off onto the substrate during film deposition as particles, they cause to be disconnect circuits of the wiring and result in the reduction of the yield of manufactures products.
For this reason, apart from the film-depositing process, optimum cleaning is carried out after processing the prescribed number of substrates, said cleaning being performed using particular cleaning gases according to differences in the plasma-forming method and structures and compositions of the deposited materials. The cleaning of this type of CVD device is an important process, as is the film-deposition process in the implementation of stabilized operation of the CVD system without exposing the interior of the depositing chamber to the atmospheric ambient.
An object of the present invention is to provide an optimum cleaning process for the CVD system disclosed in Patent Application Serial Number H11-157692.
In the manufacture of large liquid crystal displays in which low temperature polysilicon-type TFTs are used, the CVD device disclosed in Patent Application Serial Number H11-157692 uses plasma and deposits silicon oxide film on a large-area substrate using material in gas form, such as silane, in order to form at low temperatures a suitable silicon oxide film as a gate insulator film. An appropriate cleaning method is proposed which is suitable for this disclosed CVD system and a method for cleaning the CVD system is proposed in which the generation of particles is sufficiently suppressed, high manufacturing-product yield by means of said CVD system is maintained and said CVD system_can carry out stable operations without exposing the interior of the depositing chamber to the atmospheric ambient.
A method of cleaning a CVD device according the present invention can be used in the CVD system disclosed in Patent Application Serial Number H11-157692. According to one aspect of the present invention with an electrically conductive partition plate placed at ground potential, cleaning gas is fed into a plasma-generating space, active species are generated by applying high-frequency electric power to the high-frequency electrodes arranged in said plasma-generating space, said generated active species are fed into a film-deposition processing space through a plurality of through-holes in said electrically conductive partition plate and said film-deposition processing space is cleaned by means of said active species fed into this film-deposition processing space.
That is to say, in the CVD system which is disclosed in Patent Application Serial Number H11-157692, the plasma-generating space and the film-deposition processing space are separated from one another by an electrically conductive partition plate and a plurality of through-holes is made to pass from the plasma-generating space to the film-deposition processing space in said electrically conductive partition plate. The through-holes are formed such that they fulfil conditions which prevent back-diffusion to the plasma-generating space side of the material in gas form fed from the film-deposition processing space.
Cleaning gas is fed directly into the plasma-generating space, which is separated from the film-deposition processing space by the electrically conductive partition plate, and active species (radicals) are generated by applying high-frequency electric power to the high-frequency electrode inside said plasma-generating space. The generated active species (radicals) are fed into the film-deposition processing space through the plurality of through-holes in the electrically conductive partition plate, which is at ground potential, and the film-deposition processing space is cleaned by means of the active species fed into the film-deposition processing space.
According to the present invention, it is possible to use fluoride gas as the cleaning gas. One or more types of the fluoride gases from such as, for example, NF3, F2, SF6, CF4, C2F6, C3F8 can be used.
When fluoride gas is used as the cleaning gas to apply the present invention to the actual cleaning, after processing of a prescribed number of silicon oxide films and a-Si films, fluoride gas is fed into the plasma-generating space, and active species (fluorine radicals) are generated by striking electrical discharge in the plasma-generating space. The fluorine radicals are fed into the film-deposition processing space through the plurality of through-holes in the electrically conductive partition plate at ground potential, and the film-deposition processing space is cleaned. In other words, deposits attached to the inner walls of the vacuum vessel and to the surface of the substrate-holding mechanism, and the like, react with said fluorine radicals and thus can be removed and expelled from an exhaust port.
In this respect, oxygen gas can be added to the above mentioned fluoride gas in order to further the dissociation into the fluorine atom radicals. For example, J. Appl. Phys. Vol. 52 (1981) p. 162 proposes that by adding oxygen with a concentration of 60% or less, it is possible to increase the density of fluorine atom radicals in comparison with cases in which there is no additive.
If fluoride gas is used, as mentioned above, the radicals generated inside the plasma-generating space are fluoride radicals or fluorine atom radicals. However, but in cases where the deposits on the film-processing space and the like are carbonates, O2 is used as the cleaning gas.
In addition, in cases in which the density of the plasma is low and a sufficient cleaning speed is not obtained, if an inert gas with a high ionization potential such as He, Ne, Ar, Kr and Xe is admixed with the cleaning gas, it is possible to raise the temperature of the electrons by the admixture of said inert gas, to further the dissociation of the cleaning gas such as fluoride gas and to increase the cleaning speed.
In cases in which the method of cleaning a CVD system according to the present invention is implemented using fluoride gas as the cleaning gas as mentioned above, the cleaning gas which is adsorbed in the inner face of the through-holes, in the partition plate and on the partition plate during the cleaning step may desorb in the progress of the film-depositing step after the completion of the cleaning, may be discharged into the film-deposition processing space from the interior of the partition plate, and fluorine which is produced due to cleaning gas may be included in the thin film during the film deposition after the completion of the cleaning and degrades the intrinsic properties of the thin film.
The present application proposes a method of cleaning a CVD system which, as mentioned above, can suppress in advance the above_mentioned problem which occurs in cases in which fluoride gas is used as the cleaning gas in the cleaning method according to the present invention.
In the method of cleaning the above_mentioned CVD system, according to one aspect of the present invention, when, with the electrically conductive partition plate at ground potential, cleaning gas is fed into said plasma-generating space, active species are generated by applying high-frequency electric power to the high-frequency electrode arranged in the interior of said plasma-generating space and said generated active species are fed into said film-deposition processing space through the plurality of through-holes in said electrically conductive partition plate. The electrically conductive partition plate is heated, and more specifically, heating of said electrically conductive partition plate can be carried out within a temperature range which suppresses the adsorption of fluorine onto the inner circumferential face of said through-holes and the surface of the partition plate.
The temperature range at which the adsorption of fluorine onto the inner circumferential face of the through-holes and the surface of the partition plate is prevented varies respectively depending on the type of fluoride gas used as cleaning gas. For example, in cases in which the cleaning gas is a fluorocarbon gas such as CF4, C2F6, C3F8 and in cases when the cleaning gas is a nitrogen fluoride gas such as NF3 the electrically conductive partition plate is heated to 200xc2x0 C. or more, and in cases when the cleaning gas is a fluorosulfur gas such as SF6 the electrically conductive partition plate is heated to 100xc2x0 C. or more.
Such heating of the electrically conductive partition plate can be carried out, for example, by housing heating means, such as a heater, in the electrically conductive partition plate.
With the respective cleaning method, as it is possible to heat the electrically conductive partition plate to the necessary temperature at which the adsorption of cleaning gas onto the inner circumferential face of said plurality of through-holes provided in said electrically conductive partition plate and the surface of the partition plate is inhibitted, said heating being in accordance with the type of fluoride gas used as cleaning gas, it is possible to remove the fluorine which is absorbed onto the inner circumferential face of the through-holes and the surface of the partition plate during the cleaning and to prevent in advance the fluorine contamination of the thin film during the film deposition of the film-depositing process after the completion of cleaning.